To unlock the Hell Realm Cloak, you must destroy five Memory Stones. Here are the locations of the five stones. Keep in mind that all enemies in a location should be destroyed; otherwise, the stone you seek may not appear.

1.) Hill of Brimstone 1 (I)
2.) Hill of Brimstone 1 (II)
3.) Condemnation Tower (Level 2, East)
4.) Condemnation Tower (Level 3, West)
5.) Hill of Brimstone 2

After all five stones are destroyed, a sixth stone will appear. Head to the Inquisition Bridge to find it. Destroy the sixth and final stone, and the Hell Realm Cloak will be yours.